1. Field of the Invention
The present invention relates to a method for fabricating semiconductor devices, and more particularly to a method for forming accurate micro patterns having a micro dimension smaller than the resolution of a stepper used, on a semiconductor substrate.
2. Description of the Prior Art
Recently developed semiconductor devices have an increased integration degree so that they are able to contain a large number of circuits, thereby being capable of processing and storing an increased quantity of information. The integration of such semiconductor devices depends on the accuracy in forming circuits and lines connecting those circuits together within a limited region. In order to accurately form such circuits and connecting lines, it is required to form photoresist film patterns having a micro dimension. Such photoresist film patterns are used as etch barriers upon conducting an etch process required to form circuits and connecting lines.
Typically, the formation of such photoresist film patterns involves coating with a photoresist film, light exposure and development. The light exposure process is carried out using a photolithograpy device or stepper which serves to irradiate light onto the surface of the photoresist film in such a manner that the photoresist film is selectively exposed to the irradiated light by a light exposure mask. The stepper serves as an important factor for defining micro patterns having a micro dimension. The ability of such a stepper to form a micro pattern is called "a resolution". The resolution R of a stepper can be expressed by the following equation: EQU R=k.times..lambda./NA
where, k represents a process constant, .lambda. represents the wavelength of light emitted from a light source, and NA (numerical aperture) represents a constant relating to the diameter of the aperture of light passing through a lens equipped in the stepper. However, the above-mentioned parameters, namely, the wavelength of light, the diameter of the aperture of the lens, and the process constant, can not be controlled below certain limits respectively associated therewith. For example, steppers using light sources such as G-line, i-line and excimer lasers respectively having wavelengths of 436, 365 and 248 nm have a limitation in that they have light resolutions capable of forming patterns with dimensions of about 0.5, 0.35 and 0.2 .mu.m, respectively.
Meanwhile, the light exposure mask should be provided with light shield patterns which are formed on a transparent substrate in such a manner that they have a space width larger than the light resolution of the stepper as used. This is because when the light shield patterns have a space width smaller than the light resolution of the stepper, the surface of the photoresist film may be exposed to light in a wider region than is desired, due to the diffraction of light, thereby resulting in a degradation in contrast. For this reason, known methods of forming micro patterns are problematic in that they can not form micro patterns having a dimension of 0.3 .mu.m or less by use of i-line (.lambda.=365 nm) steppers. This results in a difficulty in developing semiconductor devices of 1 Giga DRAM grade.